1. Field of the Invention
The present invention relates generally to a mobile communication system, in which a plurality of radio base station have a plurality of speech channels (hereinafter simply referred to as "channel") in common, and assignment of channel is controlled in an autonomous distributing manner per each radio base station. More particularly, the invention relates to a method and a system for assigning speech channel used in the radio base station and a radio mobile station.
2. Description of the Related Art
Conventionally, an assignment method of channel in a mobile communication system of this type assigns channel which is expected to have little possibility of causing interference in radio environment of individual place on the basis of past experience, in automonously distributed fashion, as shown in Japanese Unexamined Patent Publication No. Heisei 1-80135 (Japanese Patent Application No. Showa 62-237876) for example.
FIG. 7 is a flowchart showing the conventional speech channel assignment method. A radio base station has reference for determining order of sequence for assignment per channel. When communication with a radio mobile station becomes necessary, the radio base station reads out a channel number K having the highest reference, compares an instantaneous reception field intensity (can be interference wave) in the selected channel and usability judgment threshold value T(K) to make judgment whether the selected channel can be used or not. When judgment is made that the selected channel cannot be used, the channel in question is recognized as a channel easily causing interference to perform a process for lowering preference of the channel. Then, a channel having second highest preference is selected as a candidate channel to perform judgment that the candidate channel can be used or not. When judgment is made that the selected channel can be used, call is established through the selected channel.
The foregoing conventional channel assignment method encounters a drawbacks in that since the mobile communication system is used in a certain distance between the radio base station and the radio mobile station, even the assigned channel having high preference on the side of the ratio base station and being judged to have no interference in view of usability judgment threshold value, interference can be caused in the same channel on the radio mobile station. On the other hand, since only interference record on the radio base station side is reflected in determination of preference of the channel, spatial re-use of the channel becomes not possible. Even when interference is easily cause on the radio mobile station, such channel easily causing interference can be assigned again.